LevelDB is a fast key-value storage library written at Google that provides an
ordered mapping from string keys to string values.

Features:
- Keys and values are arbitrary byte arrays.
- Data is stored sorted by key.
- Callers can provide a custom comparison function to override the sort order.
- The basic operations are Put(key,value), Get(key), Delete(key).
- Multiple changes can be made in one atomic batch.
- Users can create a transient snapshot to get a consistent view of data.
- Forward and backward iteration is supported over the data.
- Data is automatically compressed using the Snappy compression library.
- External activity (file system operations etc.) is relayed through a virtual
  interface so users can customize the operating system interactions.
- Detailed documentation about how to use the library is included with the
  source code.

Limitations:
- This is not a SQL database. It does not have a relational data model, it does
  not support SQL queries, and it has no support for indexes.
- Only a single process (possibly multi-threaded) can access a particular
  database at a time.
- There is no client-server support builtin to the library. An application that
  needs such support will have to wrap their own server around the library.

WWW: http://code.google.com/p/leveldb/